Air Stable PbSe Colloidal Quantum Dot Heterojunction Solar Cells

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Air Stable PbSe Colloidal Quantum Dot Heterojunction Solar Cells: Ligand Dependent Exciton Dissociation, Recombination, Photovoltaic Property and Stability Yaohong Zhang, Chao Ding, Guohua Wu, Naoki Nakazawa, Jin Chang, Yuhei Ogomi, Taro Toyoda, Shuzi Hayase, Kenji Katayama, and Qing Shen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10920 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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The Journal of Physical Chemistry

Air Stable PbSe Colloidal Quantum Dot Heterojunction Solar Cells: Ligand Dependent Exciton Dissociation, Recombination, Photovoltaic Property and Stability Yaohong Zhang,† Chao Ding,† Guohua Wu ,‡ Naoki Nakazawa,† Jin Chang,§ Yuhei Ogomi,ǁ Taro Toyoda,†,⊥ Shuzi Hayase,ǁ,⊥ Kenji Katayama,∮ Qing Shen*,†,⊥ †

Faculty of Informatics and Engineering, The University of Electro-Communications, 1-5-1

Chofugaoka, Chofu, Tokyo 182-8585, Japan. ‡

College of Science and Technology, Nihon University, 1-8-14 Kanda Surugadai, Chiyoda-ku,

Tokyo 101-8308, Japan. §

Institute of Advanced Materials, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009,

China. ǁ

Faculty of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4

Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0196, Japan. ⊥

CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama

332-0012, Japan.

ACS Paragon Plus Environment

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Department of Applied Chemistry, Chuo University, 1-13-27, Kasuga, Bunkyo, Tokyo 112-

8551, Japan. Corresponding Author: Qing Shen *,†,⊥ E-mail: [email protected]; Fax: +81 42 443 5501; Tel: +81 42 443 5471.


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ABSTRACT

We fabricated the long term air stable PbSe colloidal quantum dots (CQDs) based planar heterojunction solar cells (FTO/TiO2/PbSe/Au) with relatively larger active area (0.25 cm2) using tetrabutylammonium iodide (TBAI, I-) as ligand in solid state ligand-exchange process. For the first time, we have achieved the whole preparation process of the device in the ambient atmosphere from PbSe CQDs collection to PbSe colloidal quantum dot solar cells (CQDSCs) fabrication, then storage and in their following measurements. Especially, TBAI-treated PbSe CQDSCs exhibited a power conversion efficiency (PCE) of 3.53% under AM 1.5 G in air, and also a remarkable long term stability (more than 90 days) of their storage in ambient atmosphere has been identified. By contrast, 1,2-ethanedithiol (EDT), 3-mercaptopropionic acid (MPA) and cetyltrimethylammonium bromide (CTAB, Br-) treated PbSe CQDSCs were further studied. The ligand-dependent exciton dissociation, recombination, energy level shift and air stability of PbSe CQDs treated with these different ligands were systematically investigated. It was noted that TBAI-treated PbSe CQDSCs exhibited suppressed recombination, faster charge transfer rate, and longer carrier lifetimes, which resulted in a higher power conversation efficiency (PCE) and long term air stability.


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INTRODUCTION Colloidal quantum dots (CQDs) have attracted immense attention in the past decades owing to their unique size-dependent properties and low-cost solution process ability.1-5 This makes CQDs promising in various applications such as light emitting diodes (LEDs),6-7 field effect transistors (FETs),3, 8-10 biolabeling,11-13 lasers14-16 and solar cells.17-20 Recently, colloidal quantum dot solar cells (CQDSCs), which are facile to prepare by simple spin-coating procedure with low fabrication cost, have attracted increasing scientific and industrial interests as a promising candidate for the next generation solar cells. Among them, PbSe CQDs present high extinction coefficients, and long wavelength absorption spectra which can be conveniently tuned by controlling their size. Specifically, the demonstration of the multiple exciton generation (MEG) phenomena in CQDs opens the possibility of obtaining quantum efficiencies higher than 100%, that is, more than one electron generated per absorbed photon at a broad wavelength range across the solar spectrum.21-24 As predicted by Nozik and co-workers, the theoretical power conversion efficiency (PCE) of QDSCs can achieve as high as 44% due to MEG which is higher than the theoretical efficiency of single junction solar cells (Shockley Queisser limit).25 Recently, PbS CQDSCs have been reported with certified PCE of 10.8% and Zn-Cu-In-Se QDs sensitized solar cells have achieved a certified PCE as high as 11.6%.26-27 However, the efficiencies are still much smaller than the theoretical efficiency. Therefore, fundamental studies on the mechanism for improving the photovoltaic properties of CQDSCs are of great importance and necessary. Specifically, PbSe QDs have attracted attention because of their small bulk bandgap (0.26 eV),28 high dielectric constant (εm = 23),29 and large exciton Bohr radius (46 nm) which is two times higher than that of PbS (23 nm).30-31 What’s more, it has been reported that PbSe QDs have more efficient MEG than PbS QDs due to their slower hot carrier cooling rate,32 and PbSe


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CQDSCs has been confirmed with a peak external quantum efficiency of 114%.1 Unfortunately, PbSe CQDs are highly susceptible to oxidation which results in uncontrollable changes in their optical and electronic properties,30, 33 thus making the performance of PbSe CQDSCs tend to quickly degrade after the solar cells exposed to air environment. Therefore, the fabrication process and the necessary measurements for PbSe CQDSCs must be in the air free environment as reported up to now. It is no doubt that air stability is a major constraint factor on the development of PbSe CQDSCs. Recently, halide treatments have been identified as a viable method to improve the air stability of PbSe CQDs colloidal solution and film.30-31,

33-34

For PbSe CQDSCs, Asil et al.

injected cadmium chloride (CdCl2) into PbSe reaction solution after PbSe CQDs growth period to form a PbCl2 surface passivation layer outside PbSe, and obtained air stable PbSe CQDs and solar cells. But their solar cells were encapsulated in the devices fabrication process, so these solar cells are not really exposed to air.35 Beard’s group used Pb-halide precursors as the source of Pb2+ to get air stable halide ligand treated PbSe CQDs through complicated cation-exchange reactions.36-38 The best PCE of the PbSe CQDSCs in his group was reached 6.47% and the solar cell had a 5.9% PCE after 50 days of storage in air.37 But all of these solar cell devices were measured in an inert atmosphere not in air. So far, PbSe CQDSCs have not been implemented the whole process in ambient atmosphere: from PbSe CQDs washing step to solar cells fabrication, storage and measurements. In addition, charge transfer and recombination mechanism in PbSe CQDSCs are also not clear. In this paper, we described an improved simple method for fabricating air stable PbSe heterojunction CQDSCs using rarely reported tetrabutylammonium iodide (TBAI) as ligand source in solid state ligand-exchange process. The solar cells with simple structure (TiO2


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compact layer/PbSe/Au) and relatively larger active area (0.25 cm2, a mask of 0.16 cm2 was used when measuring photovoltaic performance) compared to most of the reported PbSe CQDSCs (about 0.01-0.06 cm2) were fabricated by spin-coating method in ambient atmosphere.23, 31, 37 The PCE of TBAI-treated PbSe CQDSCs was over 3.5% which measured in air, and the solar cells possessed a remarkable long term stability of more than 90 days of storage in ambient atmosphere. We compared TBAI with the other three short ligands, i.e., 1,2-ethanedithiol (EDT), 3-mercaptopropionic acid (MPA) and cetyltrimethylammonium bromide (CTAB), to investigate the ligand-dependent air stability, energy level shift, the exciton dissociation, and photovoltaic properties of PbSe CQDSCs. In addition, the charge transfer rate, recombination processes and carrier lifetimes in these CQDSCs were also revealed through ultra-fast transient absorption (TA) spectra, and open-circuit transient voltage decay measurements. EXPERIMENTAL Materials. Lead(II) oxide (PbO, Wako, 99.5%), oleic acid (OA, Aldrich, 90%), 1-octadecene (ODE, Aldrich, 90%), cadmium chloride (CdCl2, Wako, 95%), tetradecylphosphonic acid (TDPA, Aldrich, 97%), Oleylamine (OLA, Aldrich, 70%), Titanium diisopropoxide bis(acetylacetonate) (Aldrich, 75 wt.% in isopropanol), 1,2-ethanedithiol (EDT, Aldrich, 98%), 3-mercaptopropionic acid (MPA, Aldrich, 99%), cetyltrimethylammonium bromide (CTAB, Wako, 98%), tetrabutylammonium iodide (TBAI, Wako, 98%), Tetrachloroethene (TCE, Wako, 99%). These materials were used as received from commercial sources without any purification. Synthesis of PbSe CQDs. PbSe CQDs were synthesized following a similar literature method,35 but modified as a simple process in our paper. Briefly, 6 mmol PbO and 15 mmol OA were mixed with 30 mL ODE in a 100 mL three-neck flask. The mixture was stirred and degassed at room temperature for 0.5 h and then at 100°C for 1 h. The solution was then heated


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to 130°C under nitrogen for another 2 h. 18 mL of 1 M TOP-Se solution was then rapidly injected to the above lead oleate solution at 90°C. After 3 min, the heater was removed immediately while stirring of the solution was maintained. When the solution was cooled to 75°C, a CdCl2-TDPA-OLA solution containing 1 mmol CdCl2, 0.1 mmol TDPA, and 3 mL OLA was injected into the colloidal PbSe solution. After quickly cooling down to room temperature, the PbSe CQDs were isolated from the reaction solution by using acetone/methanol/hexane solvent system in air, and this purification process was repeated for three times. After purification, the obtained PbSe CQDs precipitate was then dried by an air flow and dispersed in octane at a concentration of 50 mg/mL and stored in ambient atmosphere. PbSe QDs Film Fabrication. The PbSe CQDs were deposited on glass substrates by a typical layer-by-layer spin-coating method using a fully automatic spincoater.39 Each cycle was consisted of three steps: PbSe deposition, ligand exchange, and solvent rinse. Generally, colloidal PbSe (100 µL) was dropped onto glass substrates and spun-cast at 2500 rpm for 15 s. Then, the ligand solution (0.02% vol EDT in acetonitrile, 30 mM MPA in methanol, 30 mM CTAB (as Br- source) in methanol, and 30 mM TBAI (as I- source) in methanol) was dropped onto the substrate and spun dry after a 60 s wait. The substrate was then rinsed three times with methanol (acetonitrile for EDT) to remove excess unbound ligands. Photovoltaic Device Fabrication. To fabricate PbSe CQDSCs, fluorine-doped tin oxide (FTO) patterned glass substrates were cleaned through sequential ultrasonic treatment with ethanol, acetone, isopropanol and deionized water. The washed FTO glass substrates were further cleaned with oxygen plasma for 15 min before use. FTO/TiO2 substrate was made by spinning 0.3 M Titanium diisopropoxide bis(acetylacetonate) in 1-butanol solution on FTO substrate. In other words, 200 µL Ti4+ solution was dropped on FTO glass substrates (2.5×2.5


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cm) and spin-coating at 3000 rpm for 30 s, followed by annealing at 450°C for 30 min in air. The PbSe CQDs were deposited on FTO/TiO2 substrates by spin-coating method same as PbSe films fabrication. Finally, 100 nm Au top electrode was deposited onto the PbSe layer by thermal evaporation through a shadow mask to create four identical cells on each substrate, each solar cell with an active area of 0.25 cm2. Characterization. The UV-vis-NIR absorption spectra of PbSe CQDs solution and the ligands treated PbSe QDs films were recorded using a spectrophotometer (JASCO, V-670). The Fourier transform infrared absorption (FT-IR) spectra (Thermo Scientific, Nicolet 6700) of the PbSe QDs films were measured to verify that the as-synthesized ligand (OA) on PbSe QDs were successfully exchanged. Photoluminescence (PL) measurement was conducted using LabRAM HR-800 UV. Ultra-fast transient absorption (TA) spectra were examined by using a fs-TA system. The laser source was a Ti/sapphire laser (CPA-2010, Clark-MXR Inc.) with a wavelength of 775 nm, a pulse width of 150 fs, and a repetition rate of 1 kHz. X-ray photoelectron spectroscopy (XPS) measurement was conducted to identify the presence of short ligands within the samples after ligand exchange. The measurement was carried on JEOL JPS9200. The photoelectron yield spectra of the PbSe film were recorded using an ionization energy measurement system (Model BIP-KV205, Bunkoukeiki Co, Ltd). The morphology of solar cell was examined using a scanning electron microscope (SEM, JEOL, JSM-6340). The sizes and distance between ligand treated PbSe QDs was determined by transmission electron microscope (TEM, JEOL, JEM-2100F). The current density–voltage (J–V) measurements were performed using a Keithley 2400 source meter in the dark and under AM 1.5 G irradiation (100 mW cm−2), with a Peccell solar simulator PEC-L10. The transient open-circuit voltage decay measurements were carried out using a pulsed YAG laser with a wavelength of 532 nm, a pulse duration of 5 ns


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and a repetition rate of 4 Hz. The voltage responses were recorded using an Iwatsu digital oscilloscope DS-5554. The transient voltage decay measurements were taken without a background light bias. RESULTS AND DISCUSSION

Figure 1. Transmission electron microscopy (TEM) image of PbSe CQDs (a). Absorption spectra of PbSe CQDs dispersed in TCE solution measured soon after preparation and after storing in air for 30 days (b). FTIR spectra of PbSe QDs films capped with different ligands (c). Optical Properties, Stability and Assembly of PbSe QDs. PbSe CQDs with OA capping ligands were synthesized by a simple method was shown in the experimental section. The average size of PbSe CQDs is approximately 2.9 nm in diameter, as shown in Figure 1a. Figure 1b shows optical absorption spectra of the monodisperse PbSe CQDs in TCE. The first exciton absorption peak of PbSe CQDs in TCE is about 910 nm, which corresponds to its band gap energy of 1.37 eV. After stored in air 30 days, there was not any significant change in the optical absorption spectra for PbSe CQDs TCE solution, indicating that CdCl2 surface passivation technique is an effective method to improve the stability of PbSe CQDs colloidal solution. FTIR spectra of PbSe QDs films treated with different ligands on Au coated glass substrates which are fabricated by spin coating method are given in Figure 1c. FTIR measurements demonstrated that the C-H absorption stretching peaks at 2961, 2929, and 2854 cm-1 for the films with short ligand


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treatments were significantly reduced. What’s more, the =C-H stretching peak at 3006 cm-1 belonged to OA disappeared, which suggested that OA ligands were successfully removed during ligand exchange process and replaced by short ligands. This is further confirmed by the XPS measurements. As shown in Figure S1, the clear peaks corresponding to sulfur, Br and I (S 2p, Br 3d and I 3d) could be observed, respectively. From what has been discussed above, we can draw the conclusion that PbSe QDs within the films were completely packed by short ligands after ligand exchange process.

Figure 2. Normalized absorption spectra of PbSe QDs thin films capped with difference ligands (a), and after stored in air and dark condition for 100 days (b-f).


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Figure 2a presents the normalized absorption spectra of PbSe QDs thin films capped with different ligands. The first exciton peak of PbSe-OA film was located at 1.36 eV which was similar to that of PbSe CQDs in TCE. Moreover, after ligand exchanging, the first exciton peak of MPA treated PbSe QDs film moved to 1.33 eV and PbSe QDs films treated with EDT, CTAB and TBAI exhibited their first exciton peak near at 1.32, 1.32, and 1.31 eV respectively. Compared with PbSe-OA film, the red shift of the first exciton peaks of PbSe QDs films treated with short ligands are caused by the narrowed distance among PbSe QDs (as shown in Figure S3, the average QD-QD distances were calculated as 3 nm, 1 nm, 1 nm, 0.5 nm and 0.5 nm for OA, EDT, MPA, CTAB, and TBAI ligands, respectively),40 which leads to strengthened dipoleinduced dipole interaction, enlarged wave function delocalization and increased dielectric constant of the surrounding medium, in turn results in enhanced packing densities and conductivities of the PbSe QDs films.8, 41-42 We examined the effect of air exposure on PbSe QDs thin films to investigate the stability of these thin films, which were stored in air and under dark condition after fabrication. It’s obvious that after 100 days the first exciton absorption peak of PbSe-OA thin film was blue shifted together with slightly broad, as shown in Figure 2b. This observation may originate from the changes in the surfaces of PbSe QDs which were oxidized to be higher bandgap species, such as Pb-O, or PbOH, resulting in smaller effective size of PbSe QDs (Figure S2).33, 35 It means that the surface atoms of PbSe QDs which uncapped by OA and CdCl2 are oxidized when they are exposed in air for a long time, thus the surface of PbSe-OA QDs needs for further passivation taking account of QDs’ stability. In contrary, as shown in Figure 2c and d, the first excion peak of PbSe-EDT film shows no significant change while that of PbSe-MPA film shows blue-shift. This indicates that PbSe-EDT film is more stable than PbSe-MPA film. EDT molecule with two


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thiol groups may effectively take more OA away from the PbSe CQDs surface and passivate more Pb ions on the surface of PbSe QDs than MPA molecule with only one thiol group. This is also confirmed by XPS result (Figure S2), in which, small peaks belonged to Pb(OH)2 are observed in Pb 4f region of PbSe-MPA film. Fortunately, there are no sizable peak shifts for PbSe-CTAB and PbSe-TBAI films after storing in air for 100 days, as shown in Figure 2e-f. Therefore, after CTAB and TBAI treatments, the air stabilities of PbSe QDs films are significantly improved. In Figure S2d and e, the peaks belonged to Pb oxide species were not found. And the peaks of Pb 4f7/2 (or Pb 4f5/2) for both PbSe-CTAB and PbSe-TBAI films can fit with two components: one peak of Pb 4f7/2 at 137.5 eV corresponds to the binding energy of PbSe and the other peak of Pb 4f7/2 corresponds to Pb-X bond (X is halogen). For PbSe-CTAB, the latter peak at 137.9 eV corresponds to Pb-Br bond, while for PbSe-TBAI, the latter peak at 137.8 eV corresponds to Pb-I bond. We suppose the treatment of CTAB and TBAI can form strong protective layer (Pb-Br and Pb-I) on PbSe QDs surface, which can resist the oxidation of PbSe QDs, thereby improving the stability of PbSe QDs at ambient atmosphere.


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Figure 3. Schematic energy level diagrams of PbSe QDs treated with short ligands (the first exciton peak position of the film is taken as the band gap). Ligand Dependent Energy Levels of PbSe QDs. Changing the identity of the chemical binding group and dipole moment of the ligand should also affect the strength of the QD-ligand surface dipole and shift the vacuum energy, in turn change the valence band (VB) and conduction band (CB) position of QD.43 The valence band maximum (VBM) positions of PbSe QDs thin films treated with different ligands were measured by using photoelectron yield spectroscopy (PYS, OA ligand is too insulating to be employed in PYS). Because of the collective contributions of the QD-ligand interface dipole and the intrinsic dipole moment of the ligand molecule itself,43 the VBM positions of PbSe QDs treated with organic ligands EDT and MPA upward shift to -4.86 eV and -4.92 eV in comparison with halide ligands CTAB (-5.2 eV) and TBAI (-5.2 eV), respectively (as shown in Figure S4). The energy level diagrams of PbSe QDs treated with ligands were estimated from the PYS and UV-vis-measurements as shown in Figure 3. These energy level shifts will significantly influence the charge injection efficiency and open-circuit voltage of photovoltaic devices.


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Figure 4. Steady state PL spectra measured at 77 K (a) and 298 K (b) for PbSe QDs films treated with ligands (the PL intensity of PbSe-OA, PbSe-EDT and PbSe-MPA films are shrunk 20 times, 5 times and 5 times than their original values, respectively). Ligand Dependent Exciton Dissociation in PbSe QDs. Surface ligands treatment not only can influence the distance between QDs (Figure S3) and energy levels of QDs but also has effects on the QDs surface trap density and charge transfer between QDs.44-45 In order to avoid formation of oxidation of PbSe QDs, PL spectra of PbSe QDs thin films capped with different ligands were measured under vacuum at 77 K and 298 K with the excitation wavelength of 532 nm. It is known that the relaxation process of photoexcited carriers in QD solids usually includes radiative and non-radiative recombination.46 The PL corresponds to the radiative recombination of excitons in the QDs. The PL quenching of QD solids is highly associated with non-radiative recombination. The non-radiative recombination of charge carriers in QDs solids are usually


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through four routes: carrier trapping by defects in QDs, charge transfer between QDs (i.e., the exciton dissociation), charge transfer from QDs to ligands which usually occurs when the ligand has conjugated structure, and energy transfer between QDs which usually occurs when the QDs have wide size distribution. In our work, all of the short ligands do not have conjugated group and PbSe QDs have a narrow size distribution. Thus, the PL quenching is considered to mainly depend on the trap density of PbSe QDs and the charge transfer between PbSe QDs. As shown in Figure 4a, the PL emission peak of PbSe-OA is at 1.11 eV. For the short ligands treated PbSe QDs films, the emission peaks are red shifted to 1.09 eV, 1.07 eV, 1.06 eV and 1.05 eV, respectively, the trends are consistent with the absorption spectra of the films. And the PL intensity decreased with the ligand length becoming shorter. Photoexcited carriers in PbSe-OA QDs with longest dielectric OA ligands between QDs are difficult to transfer to the adjacent QDs, which leads to the largest PL emission. For short organic ligands EDT and MPA capped PbSe QDs, the PbSe inter-QD coupling is enhanced than that of PbSe-OA, so some photogenerated carriers can transfer to the adjacent QDs, some are trapped by surface or deep defects (we have observed surface oxidation by XPS for MPA treated PbSe QDs as mentioned earlier) and others are recombined through radiative recombination way. What’s more, larger PL quenching is found in the CTAB and TBAI treated PbSe QDs solid films (high packing density) compared with PbSe-EDT and PbSe-MPA films. This phenomenon can also be observed at 298k (Figure 4b). As we discussed above, the short ligands can reduce the surface trap density of PbSe QDs, in turn improve the stability of PbSe QDs, especially for PbSe-CTAB and PbSe-TBAI. Therefore, the PL quenching of PbSe-CTAB and PbSe-TBAI films mainly depends on the charge transfer among PbSe QDs (the exciton dissociation). Thus, we can believe that the charge transfer from QD to QD (the exciton dissociation in QDs) in CTAB and TBAI treated PbSe QDs


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solid are faster and more unobstructed than those PbSe QDs solid treated with organic ligands. Next we demonstrate this consideration by measuring the charge transfer rate from QD to QD (i.e., exciton dissociation rate) for different ligands treated PbSe QDs films using ultra-fast TA spectra measurement.

Figure 5. Comparison of the TA decays of PbSe QD films with different ligands. All samples were pumped by 500 nm laser pulse with the pump fluence of 6 µJ/cm2, and probed at 935 nm. The solid lines are fitting curves with one exponential equation. In all of the TA measurements, the pump light wavelength was 500 nm and the probe light wavelength was 935 nm (the peak position of the optical absorption spectra of the PbSe QD films as shown in Figure 2). Thus, the TA signal corresponded to the bleaching between LUMO and HOMO in the QDs. Then the TA signal intensity is proportional to the photoexcited exciton


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density in QDs.47 Taking into the consideration of two-carrier and three-carrier recombination processes in the QDs, TA decay curves can be represented by the following equation:48 ⁄ = + + () where the first term An represents single-carrier behaviour, the second term Bn2 represents two-carrier (electron-hole) radiative recombination (i.e. PL emission), and the third term Cn3 represents the three-carrier Auger recombination process. In order to particularly evaluate the charge transfer process, i.e., the exciton dissociation (which is a single-carrier behaviour) in PbSe QD films, two-carrier and three-carrier recombination processes are needed to avoid which tend to take place under strong pump fluence (as shown in Figure S5). Therefore, a weak enough pump fluence of 6 µJ/cm2 was applied here, under which condition the normalized TA decay can be fitted very well with one exponential decay, i.e., the first term of eq. (1). In Figure 5, for PbSe-OA film where QDs are separated by OA ligands, the TA signal is almost constant and no decay can be observed on the time scale of 1 ns. This result indicates that charge transfer between the QDs does not occur when the QD-QD distance is large enough (about 3 nm here). For short ligands treated PbSe films, the TA spectra can be well fitted by one exponential decay with a decay time τ. As shown in Figure 5, the values of τ in PbSe-EDT, PbSe-MPA, PbSeCTAB and PbSe-TBAI are about 2.43 ns, 0.47 ns, 41.45 ps and 50.82 ps, respectively. The corresponding charge transfer rates kct=1/τ (i.e., the exciton dissociation rate in PbSe QDs) of PbSe-EDT, PbSe-MPA, PbSe-CTAB and PbSe-TBAI are 4.11 x 108 s-1, 2.13 x 109 s-1, 2.41 x 1010 s-1, 1.97 x 1010 s-1, respectively. It means that kct in PbSe-CTAB and PbSe-TBAI solid QD films are 1-2 orders of magnitude larger than that in PbSe-EDT and PbSe-MPA, which is consistent with the PL quenching results. According to above experimental results, it can be concluded that the halide ligands capped PbSe QDs have faster exciton dissociation rate than


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those of short organic ligands capped. The reason can be considered as follows. The exciton dissociation (charge transfer from QD to QD) in the ligand treated PbSe QDs films occurs through electronic tunnelling effect.49-50 So the charge transfers rate kct exponentially decreased with the QD-QD distance. Thus, kct increased largely for the halide ligands capped PbSe QDs film because of the smallest QD-QD distance of about 0.5 nm. Ligand Dependent Photovoltaic Performance of PbSe CQDSCs. As is well known, the above discussion of exciton dissociation and charge transport in QDs active layer are of great value for deeply understanding the performance of photovoltaic devices. Figure 6 shows the light and dark current density-voltage (J-V) curves of PbSe CQDSCs. It is found that devices using EDT and MPA as surface ligands exhibit higher open-circuit voltage (Voc) than those using CTAB and TBAI as ligands. The reason is not very clear now. One possibility is considered as follows. It was reported that EDT and MPA treated PbSe and PbS QDs exhibited p-type behaviour and Br- and I- treated PbSe and PbS QDs with near n-type characteristics,8, 30, 37, 40, 50-53 so the difference between the Fermi energy level (EF) and VBM in PbSe-EDT and PbSe-MPA are smaller than those in PbSe-CTAB and PbSe-TBAI. Despite the energy levels of PbSe-EDT and PbSe-MPA are higher than PbSe-CTAB and PbSe-TBAI (as shown in Figure 3), the EF of PbSe-EDT and PbSe-MPA QDs may be lower than those of PbSe-CTAB and PbSe-TBAI.36 So the EF differences of TiO2 and PbSe-organic ligand are larger than that of TiO2 and PbSe-halide ligand, which results in higher Voc of PbSe-EDT and PbSe-MPA CQDSCs.


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Figure 6. (a) J-V curves of PbSe CQDSCs using different ligands (measured in air), (b) photograph and (c) SEM cross-section image of PbSe planar heterojunction CQDSCs. In contrary, the short-circuit current density (Jsc) of both of EDT and MPA treated devices are lower than that of devices treated with halide ligands. As we discussed above, the exciton dissociation rate, i.e., the charge transfer rate in the EDT and MPA treated QD films are 1-2 orders smaller compared to those in halide ligand treated the QD films. Larger PL intensity was observed for the former QD films. Therefore, one reason for the lower Jsc of PbSe-EDT and PbSe-MPA devices could be attributed to the relatively smaller exciton dissociation rate (charge transfer rate). It is worth noting that CTAB and TBAI exhibit huge distinct influence on the performance of PbSe CQDSCs. The PCE of PbSe-TBAI devices (3.50%) shows about 3.5 times value of PbSe-CTAB device (1.04%), which is mainly ascribed to the high Jsc and FF in PbSe-


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TBAI device. We have known that the bonding energy between Pb2+ and I- is stronger than that between Pb2+ and Br-, which leads to less surface trap density on PbSe-TBAI QDs.30, 54 Surface traps can capture the photoexciton carries and as the electron-hole recombination centres, which play a destructive impact on the charge collection efficiency and thus Jsc of CQDSCs. As we discussed earlier, the exciton dissociation rates are almost the same for PbSe-CTAB and PbSeTBAI QDs films. Thus, the difference in the Jsc and FF in the two kinds of CQDSCs would be resulted from the difference in charge recombination and the charge collection efficiency. Moreover, Zhang et al investigated the hot carrier dynamics in PbSe-I and PbSe-Br QD films, they found that PbSe-I showed longer hot carrier thermalization time than PbSe-Br.54 So the relatively slow hot carrier cooling in PbSe-TBAI film has a possibility for some hot carrier injection which may result in an increase in the Jsc of device. Table 1. Performance details of PbSe CQDSCs using different ligand a Devices

Jsc (mA/cm2)

Voc (V)

FF (%)

PbSe-EDT fresh

3.48±0.22 (3.70)

0.58±0.01 (0.59)

31.6±0.4 (32.0)

0.64±0.05 (0.69)

PbSe-EDT 3 days

3.02±0.09 (3.09)

0.57±0.01 (0.58)

29.3±0.5 (29.7)

0.51±0.02 (0.53)

PbSe-MPA fresh

4.64±1.95 (6.59)

0.53±0.01 (0.54)

16.4±0.2 (16.6)

0.41±0.18 (0.59)

PbSe-MPA 3 days

2.46±0.75 (3.20)

0.52±0.01 (0.52)

15.9±0.4 (16.2)

0.20±0.07 (0.27)

PbSe-CTAB fresh

7.61±0.55 (7.06)

0.40±0.01 (0.40)

33.8±4.6 (38.4)

1.04±0.05 (1.09)

PbSe-CTAB 3 days

7.20±0.11 (7.25)

0.38±0.01 (0.38)

24.2±4.1 (28.9)

0.66±0.14 (0.80)

PbSe-TBAI fresh

18.1±0.1 (18.1)

0.42±0.01 (0.43)

44.6±0.8 (45.4)

3.50±0.03 (3.53)

PCE (%)


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PbSe-TBAI 3 days

18.3±0.1 (18.3)

0.42±0.01 (0.42)

44.9±0.5 (45.4)

3.49±0.03 (3.52)

a

Numbers in parentheses represent the values obtained for the best-performing cell. To account for experimental errors, four devices of each type were measured to give the reported averages and deviations. All of the devices were stored and measured in air.

Figure 7. (a) The open-circuit photovoltage decay curves for PbSe CQDSCs with different ligands. (b) The effective carrier lifetime calculated from the voltage decay curves. (c) Schematic illustration of the energy level alignment in the PbSe CQDSCs under different conditions. The voltage decay process is mainly through two recombination paths: (i) intrinsic trapping-assisted recombination in the PbSe QDs layer, and (ii) interfacial recombination at the TiO2/PbSe interfaces and PbSe/Au interfaces. In order to reveal the effects of different ligands on charge carrier recombination and charge carrier lifetime in PbSe CQDSCs, transient open-circuit photovoltage decay measurements were


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carried out. Figure 7a shows the transient photovoltage decay curves of PbSe CQDSCs with different ligands. It evidences that the PbSe-TBAI device exhibits much slower decay processes than those devices treated with other ligands. To quantitatively analyse the photovoltage decay processes, the decay curves can be fitted by using a dual exponential decay according to the following equation: () = ⁄ + ⁄ ( ) where A1 and A2 are proportionality constants, τ1 and τ2 are time constants.39 The fitted curves are shown in Figure 7a (solid lines) and the corresponding parameters are shown in Table S1. According to the fitting data, the voltages of the PbSe-EDT and PbSe-MPA devices decrease quickly than those of PbSe-CTAB and PbSe-TBAI. What’s more, the weight of fast voltage decay process (A1) in PbSe-EDT and PbSe-MPA devices takes up a large proportion about 59.5% and 76.3%, respectively. The faster voltage decay of PbSe-EDT and PbSe-MPA devices is mainly ascribed to their larger defect density. To make sense of charge carrier recombination processes in CQDSCs, we evaluated the recombination process based on the effective carrier lifetime (τeff), which can be defined by the following equations: 39, 55 = −(

)/( ) = /( + ) ()

=

∙ ( )

=

∙ (!)

where k is the Boltzmann constant, T is the temperature, q is the elementary charge, n is the free electron density in the TiO2 film and p is the free hole density in the PbSe QDs. τn and τp are the free electron lifetime in the TiO2 and the free hole lifetime in the PbSe QDs layer,


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respectively. According to the above equations, the open-circuit photovoltage decay is dependent on both the electron and hole lifetimes in PbSe CQDSCs. The τeff was calculated from the voltage decay curves (Figure 7a) which is shown in Figure 7b. The observed trend in τeff was TBAI > CTAB > EDT > MPA, which was consistent with the photovoltaic performances. The value of τeff in the PbSe-TBAI device is 1 or 2 orders of magnitude higher than those of other three devices, which confirms that the carrier lifetime in PbSe-TBAI device is longer than those of others. This corresponds to higher Jsc of PbSe-TBAI device due to larger charge collection efficiency. More interestingly, the photovoltage-dependent effective carrier lifetime curves can be separated into two sections, I and II, which correspond to two different recombination processes. It can obviously find that the voltage of devices mainly is influenced by the smaller τeff carriers in the solar cells. In the smaller τeff region (less than 0.7 ms, section I in Fig 7b), the values of τeff in different ligands treated PbSe CQDSCs exhibit a large difference when they get the same voltage. This confirms that the voltage decay process is dominated by the hole trapping mechanism in PbSe QDs layer (Figure 7c). The larger τeff region II (over 1 ms) is belonged to slow voltage decay process, in which the values of τeff in PbSeEDT, PbSe-MPA and PbSe-CTAB devices are nearly the same (except PbSe-TBAI). This means that the slower recombination region II mostly due to electron recombination in TiO2 and/or TiO2/PbSe interfaces, and the ligands treatments have no obvious effect on this recombination process. More details about the effect of TBAI on the interfacial recombination will be investigated in future work. Based on the above results, it can conclude that the trap or defects state in PbSe layer have great impact on the Voc of CQDSCs, and QDs surface treatment with TBAI can significantly reduce the surface states and enhance the charge collection efficiency in PbSe QDs layer, thus enhancing the Jsc, Voc and PCE of PbSe CQDSCs.


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Figure 8. Stability evaluation of PbSe-TBAI CQDSCs. The solar cells were stored and tested in an ambient atmosphere. Besides efficiency, the stability is also a significant indicator for the overall evaluation of solar cells. As shown in Table 1 and Figure S6, PbSe-TBAI device is the most stable one after 3 days while PbSe-EDT, PbSe-MPA and PbSe-CTAB devices show more or less reduction. Furthermore, we have evaluated the long term stability of the PbSe-TBAI device. All the solar cells were kept and tested in air without the control of humidity. In Figure 8, PbSe-TBAI CQDSCs exhibited the excellent stability for over 10 days and the efficiency still reaches over 75% of original after 90 days (>2000 h). What’s more, we also investigated the continuous illumination stability of PbSe-TBAI CQDSCs in air (Figure S7). Unfortunately, the Voc and FF of solar cell were gradually reduced along with the increase of illumination time, and the PCE of solar cell only keeps about 70% of the original after continuous illumination 3.5 hours (under 100 mW/cm2, AM 1.5 G illumination). This result indicates that light irradiation in air may accelerate the degradation of the PbSe CQDSCs. More detailed mechanism on the degradation and approach to suppress the degradation will be carried out.


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CONCLUSIONS In summary, long term air-stable and high efficiency PbSe CQDSCs were obtained by using TBAI as ligand in solid state ligand-exchange process, and it is realized that the whole processes: from PbSe QDs washing step to PbSe CQDSCs fabrication, storage and measurements are all in ambient atmosphere for the first time. In addition, ligand-dependent performance of PbSe QD films and CQDSCs were systematically investigated in this work. By monitoring the absorption, PL and TA spectra of the samples, we confirmed that TBAI treatment can produce more airstable and higher charge transfer rate PbSe QD films than mercapto organic short ligands (EDT and MPA). Due to less surface trap density and higher charge transfer rate in the solar cells, the PCE of TBAI treated large size PbSe CQDSCs is obtained as high as 3.53%. The mechanism behind this achievement was explored using open-circuit voltage decay. It was informed that the TBAI treatment significantly reduced the intrinsic hole trapping-assisted recombination in PbSe layer and improved the effective carrier lifetime in the PbSe CQDSCs. The device stability was also evaluated, which showed excellent storage stability in air (the efficiency of CQDSCs still remained 77% of original value over 90 days). In a word, long term stability and high efficiency PbSe CQDSCs can be fabricated and tested in ambient atmosphere by using TBAI as ligands. This work would shed light on the investigation of other PbSe QDs based devices, such as FET.

ASSOCIATED CONTENT Supporting Information. Additional information that contains the XPS results, TEM images, photoelectron yield spectra of PbSe QDs films, power dependent TA spectra of the PbSe-OA solid QD films, J-V curves of PbSe CQDSCs using different ligands after storing in air for 3 days, and J-V curves of PbSe-TBAI CQDSCs which measured under continuous illumination as


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noted in the text. This information is available free of charge via the Internet athttp://pubs.acs.org. AUTHOR INFORMATION Corresponding Author ∗ E-mail: [email protected]; Fax: +81 42 443 5501; Tel: +81 42 443 5471. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This research was supported by the Japan Science and Technology Agency (JST) CREST and PRESTO programs as well as MEXT KAKENHI grant number 26286013. REFERENCES 1.

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48. Ono, M.; Nishihara, T.; Ihara, T.; Kikuchi, M.; Tanaka, A.; Suzuki, M.; Kanemitsu, Y., Impact of Surface Ligands on the Photocurrent Enhancement Due to Multiple Exciton Generation in Close-Packed Nanocrystal Thin Films. Chem. Sci. 2014, 5, 2696-2701. 49. Gao, J.; Zhang, J.; van de Lagemaat, J.; Johnson, J. C.; Beard, M. C., Charge Generation in PbS Quantum Dot Solar Cells Characterized by Temperature-Dependent Steady-State Photoluminescence. ACS Nano 2014, 8, 12814-12825. 50. Liu, Y.; Gibbs, M.; Puthussery, J.; Gaik, S.; Ihly, R.; Hillhouse, H. W.; Law, M., Dependence of Carrier Mobility on Nanocrystal Size and Ligand Length in PbSe Nanocrystal Solids. Nano Lett. 2010, 10, 1960-1969. 51. Milliron, D. J., Quantum Dot Solar Cells: The Surface Plays a Core Role. Nat. Mater. 2014, 13, 772-773. 52. Leschkies, K. S.; Kang, M. S.; Aydil, E. S.; Norris, D. J., Influence of Atmospheric Gases on the Electrical Properties of PbSe Quantum-Dot Films. J. Phys. Chem. C 2010, 114, 99889996. 53. Zhitomirsky, D.; Furukawa, M.; Tang, J.; Stadler, P.; Hoogland, S.; Voznyy, O.; Liu, H.; Sargent, E. H., N-Type Colloidal-Quantum-Dot Solids for Photovoltaics. Adv. Mater. 2012, 24, 6181-6185. 54. Zhang, Z.; Yang, J.; Wen, X.; Yuan, L.; Shrestha, S.; Stride, J. A.; Conibeer, G. J.; Patterson, R. J.; Huang, S., Effect of Halide Treatments on PbSe Quantum Dot Thin Films: Stability, Hot Carrier Lifetime, and Application to Photovoltaics. J. Phys. Chem. C 2015, 119, 24149-24155.


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55. Zaban, A.; Greenshtein, M.; Bisquert, J., Determination of the Electron Lifetime in Nanocrystalline

Dye

Solar

Cells

by

Open-Circuit

Voltage

Decay

Measurements.

ChemPhysChem 2003, 4, 859-864.


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TOC GRAPHICS


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Air Stable PbSe Colloidal Quantum Dot Heterojunction Solar Cells

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